1. Field of the Invention
This invention relates generally to electrostatic plotting, and particularly to electrostatic color plotting using multiple color print stations to produce a full-color image in a single pass, or a single writing head to produce a full-color image in multiple passes, and to a system for detecting and maintaining the alignment of the image at each station or on each pass in such plotters.
2. Description of the Prior Art
Electrostatic printing and plotting is accomplished by placing electrostatic charge in the form of the image to be printed on an electrographic media, usually paper. A liquid toner is then applied to the paper to produce a permanent visible image. In electrostatic color plotting or printing ("plotting" and "printing" are used interchangeably herein), separate color images are individually electrostatically printed on the paper and toner applied, with each image typically corresponding to one of three colors: cyan, yellow, and magenta. Although these three colors could be combined to produce black, because a better black may be achieved using a dedicated toner station, a fourth station employing black toner often is employed in such plotters. Consequently a full color image typically requires four single color images.
Commercially available electrostatic color plotters often employ a writing head consisting of an elongate arrangement of several thousand styli, at densities of about two hundred to the inch. Electrostatic printing paper is drawn across the styli while individual ones of the styli are selected and impressed with a voltage potential that, in conjunction with another electrode mounted proximate the paper, creates a "dot" of charge on the paper. The image for each color may be considered as consisting of lines or "rasters" of these dots which, when toned, form the image of that color. For full color, the cyan, yellow, magenta, and black images are separately printed, but each is registered to be superimposed upon the preceding image. Thus, depending upon the final color desired, any particular dot location on the image may have one or more colors printed there.
In multiple pass plotters of the prior art, the full color image is printed by passing the paper through a print station, which includes a single electrostatic writing head and several different color toner baths, one color of which is employed to print the first image. The paper is then rewound and drawn past the writing head again with a different colored toner bath moved into position for each subsequent image. This multiple pass technique suffers from a number of disadvantages, including the time required to complete the total image. With respect to our invention, however, the most significant disadvantage of such multiple pass technology is the extreme difficulty of maintaining the relative position of each successive image upon the paper relative to all preceding images. Furthermore, this problem is exacerbated by changes in the dimensions of the paper itself caused by humidity, nonuniformities, and forces applied during printing.
Many of the disadvantages of multiple pass plotters have been overcome by single pass multiple station color plotters. In such plotters, the media is drawn past a succession of writing heads, each with its own corresponding toner application and drying station. Although single pass technology alleviates many of the problems of multiple pass technology, techniques must still be employed to assure that the image placed on the media at each station is in proper registration with all preceding images.
One prior art solution for maintaining proper registration of images has been to print colored alignment marks on the media around the periphery of the images. These marks are detected at downstream stations in a single pass plotter, or prior to each pass in a multiple pass plotter. In either of such systems an optical detector, such as a charge coupled device, is positioned to detect the location of the alignment marks and supply appropriate signals to physically reposition the media with respect to the writing head(s). One such prior art technique is described in U.S. Pat. No. 4,485,982.
Unfortunately, such prior art systems suffer from several disadvantages. Special apparatus must be provided to apply the toner for the alignment marks at the first station, or during the first pass of the media. Because the colors employed during the first pass at the first station in such a plotter are not necessarily the most desirable colors for the printing of the alignment marks, a separate toning station often is required to print the marks. The extra station requires extra hardware and consumes additional space in a machine which is desired to be as small as possible. The extra printing station must also be supported by a supply of toner, a pump for delivering the toner to the station, and a system for removing excess toner from station as well as the media.
A further disadvantage of such systems is the need for special mark detection hardware at each subsequent station, or in multiple pass systems at the printing station. Such additional hardware typically includes optical sensors, support circuitry and software for each. Additionally, the optical sensors at each head must be aligned with respect to that head within approximately a thousandth of an inch for sufficiently accurate use of the sensed information. Furthermore, because the alignment marks are printed onto the same media as the image, for applications in which viewing the alignment marks is undesirable, the image must be trimmed or masked to hide the marks.